1. Field of the Invention
The present invention generally relates to heating, ventilation, and air conditioning (HVAC) systems; and, more specifically, to HVAC systems that transfer sensible and/or latent energy between air streams, and humidify or dehumidify an air stream.
2. Description of the Related Art
Contemporary HVAC systems typically include high levels of ventilation and a means to recover sensible and sometimes latent energy from the exhaust air stream. Generally, energy recovery systems operate to extract heat (sensible energy) and moisture (latent energy) from one air stream and add the heat and moisture to the other air stream. For example, heat and humidity might be extracted from external air at a relatively high temperature and humidity combination, as may occur during the summer, or heat and humidity might be added to external air at a relatively low temperature and humidity combination, as may occur during the winter. The appropriate addition and removal of temperature and humidity from or to the external air not only increases comfort, but it reduces overall energy requirements for conditioning the incoming external air.
Known energy recovery systems began with plate heat exchangers, heat pipes, and sensible heat wheels. These systems were the first devices utilized to transfer sensible energy from one air stream to a second air stream. Later, the use of desiccants improved the performance of the wheel systems. In the heat wheel systems, desiccants are applied to the heat transfer surfaces enabling the transfer of both sensible and latent energy from a first air stream to a second air stream. Other known energy recovery /humidity control systems use liquid desiccants pumped from a packed bed in one air stream to a second packed bed in a second air stream to enable the transfer of latent and sometimes sensible energy from the first air stream to the second air stream.
The plate heat exchanger, as well as the heat pipe, are simple and, like common heating and air conditioning systems, almost never require maintenance over their typical 20-year life span. However, these devices do not transfer latent energy and thus have lost favor in the market. Heat wheels on the other hand, do transfer latent energy but are known to require considerable maintenance. Heat wheels experience failures of drive belts, bearings, gear drives, seals, collapse of structural members, and desiccant coating damage which includes chipping, flaking, and permanent clogging due to adsorbed contaminates. Additionally, the heat wheel""s air passageways are exceptionally small and clog quickly if the heat wheel""s air filtration system is not meticulously maintained. The desiccant coatings of the heat wheel, if they remain in place without damage, degrade in efficiency over time, as all desiccants do, but cannot be replaced. In four to six years most desiccant coated heat wheels must be replaced due to worn out mechanical parts and/or damaged or noneffective desiccant coatings. For these reasons the market has desired another device that can transfer both sensible and latent energy but one that does not have the operational problems and short life of a wheel.
Known liquid desiccant energy recovery or dehumidification systems using packed beds are typically employed only in heavy industrial applications due to their large size, initial capital cost, and high maintenance cost. Liquid desiccant systems are used in manufacturing processes due to their ability to dry air below the levels possible with other systems. Packed beds is a method to bring the liquid desiccant in direct contact with the air stream utilizing a large amount of surface area in a relatively small space. Liquid desiccant migrates to downstream areas of the system and because most liquid desiccants are very corrosive, they can cause considerable damage to downstream components of the system. In an attempt to avoid this in large part, liquid desiccant systems move air at very slow speeds or in complicated paths, giving rise to their large size. Liquid desiccant systems use components made from non-corrosive materials, which along with their large size gives rise to their high cost. Packed bed media must be replaced periodically because accumulated debris degrades effectiveness over time. Packed beds also require periodic cleaning and changing of the liquid desiccant as they act much like air washers and accumulate the debris.
A newer energy recovery system includes the use of solid desiccants that are adhered to opposing sides of a structure that is exposed to different air streams on each of its two sides. The sensible and latent energy is transferred from the desiccant material on one side, to the supporting material, then to the desiccant on the opposing side, and is released then to the second air stream. U.S. Pat. No. 5,653,115 is illustrative of such a solid desiccant approach.
Known humidification and dehumidification methods and devices can be combined with the previously described energy recovery systems as needed. Known humidification systems use packed beds to evaporate water into the air stream or use energy to heat water and then inject it as steam into the air stream. Packed beds with standing water have been identified as a source of indoor air contaminates with the growth of mold, spores and bacteria. Steam humidification equipment is expensive and uses a lot of energy to operate. Neither system is well suited for inclusion in roof mounted HVAC equipment as they deal with pure water that needs winter protection from freezing.
Known dehumidification systems use heat regenerated desiccant-coated wheels or super cool the air to condense the moisture out using vapor compression or chilled water components. Equipment that cools the air uses large amounts of energy to operate and are quite expensive to purchase and install. Equipment that uses desiccant-coated wheels have all the disadvantages of the coated wheel as described above and uses large amounts of energy to heat the desiccant in the regeneration process.
The invention relates to an enthalpy pump for altering the humidity levels of an air stream by exposing the air stream to a contained liquid desiccant. In one aspect, the enthalpy pump comprises a container having a transfer portion adapted to hold a liquid desiccant. The container comprises a conduit forming a circulation loop that will extend through a first air stream, and through a second air stream. The transfer portion is permeable to water vapor and impermeable to liquid desiccant so as to retain the liquid desiccant within the container while permitting exposure of the liquid desiccant to the air streams. The transfer portion includes a first portion to be located in the first air stream and a second portion to be located in the second air stream. The transfer portion is further adapted to move the liquid desiccant into and out of at least one of the streams whereby humidity in the air stream can be altered.
A second embodiment of the enthalpy pump further comprises a support, preferably in the form of a rotating wheel, for moving the container in and out of an air stream or between a first and a second air stream. Preferably, there are multiple blade-like containers extending from the center of the wheel radially outwardly and the wheel overlies at least a portion of first and second air streams, whereby the rotation of the wheel moves the liquid desiccant in the blade-like containers in and out of the first and second air streams.
In a third embodiment of the enthalpy pump, the container comprises a reservoir for storing a liquid desiccant and a conduit adapted to extend into the air stream, and the transfer portion is formed in the conduit.
Preferably, the transfer portion has openings sized between approximately 3 and 500 angstroms. The transfer portion is preferably made from one of PTFE, HDPE, PBA, PEN, and sintered plastics.
The enthalpy pump can have many alternative configurations. For example, the enthalpy pump can include multiple conduits, each of which extends into different air streams. A water supply can be fluidly connected to the enthalpy pump to add water and increase the concentration of the water in the liquid desiccant for increased humidification performance. Also, a water extractor can be added to the enthalpy pump to remove water from the liquid desiccant thereby decreasing the concentration of water in the liquid desiccant thereby increasing dehumidification performance. The different configurations are in part due to the many functional modes the enthalpy pump is capable of operating in. In multiple air stream systems it can operate as an enthalpy recovery system, an enthalpy recovery system with added humidification, an enthalpy recovery system with added dehumidification, a sensible recovery system, a sensible recovery system with humidification, a sensible recovery system with dehumidification, or a latent only recovery system. In a single air stream system it can operate as a humidifier system, or a dehumidifier system.
In all embodiments, means can be provided for additionally effecting sensible energy transfer to alter the temperature of an air stream.